Using a sharp knife, split the vanilla beans lengthwise, if desired. Place them in an airtight jar or bottle. (Using a knife or kitchen shears, cut the beans to the size of your vessel if necessary.) Pour enough alcohol over vanilla beans to completely cover. (Keep your container in mind when choosing bean amount, as per FDA guidelines rule of thumb; see Note.) Seal, and store in a cool, dry place, shaking and agitating every few days, for at least 3 to 4 months before using. Store for up to 2 years.